1. Field of the Invention
This invention pertains to the field of four wheel drive systems for motor vehicles, particularly to control of a transfer case clutch that distributes torque to the front and rear axles.
2. Description of the Prior Art
In a four-wheel drive 4WD system engine torque is transmitted to a secondary pair of drive wheels in response to a control signal applied to a transfer clutch located in a transfer case. The control signal is produced when certain parameters, represented by signals produced by electronic sensors, indicate vehicle operating conditions can be improved by activating both the primary drive wheels and secondary drive wheels. U.S. Pat. No. 4,757,870 describes a transfer clutch controlled with reference to the rotational speed difference between the primary and secondary drive wheels. U.S. Pat. No. 5,010,974 describes a similar system for controlling a transfer clutch upon reference to the speed difference, vehicle speed, and engine intake manifold vacuum.
Conventionally a 4WD system includes a transfer case driveably connected to the output shaft of a multiple-speed transmission. The transfer case contains the transfer clutch, a speed-reduction gearset for increasing transmission output torque, a mechanism for engaging and disengaging the gearset and a torque path, such as a ball ramp drive, for transmitting power through the transfer case to the front driveshaft through the transfer clutch.